A comparison of heart and kidney allografts in controlled histocompatible dogs is being carried out. In addition, the mixed leukocyte culture technique (MLC) is being quantitated and evaluated for its effectiveness in predicting graft survival time. Studies have shown that in MLC-identical littermates, the heart is rejected much more slowly that the kidney, maintains function longer, and shows fewer microscopic changes in a given period of time. As donor and recipient become more dissimilar (as judged by the MLC test) these differences disappear, and both grafts are rejected much more quickly. One animal, surviving 578 days with both heart and kidney grafts, shows the same function in the transplanted kidney as in his natural kidneys. With moderate immunosuppresive therapy, low MLC stimulator kidneys survived more than four times longer than high MLC stimulator kidneys when kidneys from mixed-breed dogs were placed in pure-bred beagles. This suggests that the quantitation of MLC is reliable in predicting organ survival.